High strength stainless steels provided with corrosion resistance which is one of the characteristics inherent to the stainless steel as well as considerably high strength properties. High strength stainless steels should, of course, be of high strength. But also it is most desired that they are excellent in workability and weldability including various properties of the welds, since they are generally subject to working and welding when they are used. Also it is a matter of course that they should be excellent in corrosion resistance which is one of the inherent characteristics of stainless steel. It is not easy to obtain all these properties simultaneously. (One of the difficulties is the incompatibility of strength and workability.) However, there are some fields in which high strength stainless steel materials satisfactorily provided with all of the above-mentioned properties are required. One of such fields is rolling stock.
Because of their excellent corrosion resistance, more and more high strength stainless steels are being used nowadays for rolling stock, whereas plain carbon steel was largely used in the past. Plain carbon steel is not satisfactory in that it is inferior in corrosion resistance and strength, and therefore it requires considerable cost for maintenance, such as periodical painting etc., and considerably thick plates must be used to compensate for its inferior strength, which makes the vehicles heavier. This runs counter to the current general demand to save material and energy consumption. In order to overcome these disadvantages of plain carbon steel, use of stainless steels having excellent corrosion resistance as well as high strength is desired. When stainless steel is used, the rolling stock can be made lighter by employment of thinner plates and the need for troublesome maintenance work, including painting, can be eliminated. Further, stainless steel is more durable than plain carbon steel, and its use is advantageously meets the demand for the saving of material and energy in various respects. Thus, there is now being seen a switchover of the material for rolling stock from plain carbon steel to high strength stainless steel, and this trend is expected to increase.
When railroad vehicles are constructed, cold-rolled plates of various thicknesses are formed into complicated shapes, and therefore, the plates must be of high strength and at the same time must be provided with good ductility and workability in the cold-rolled state. Further, the shaped plates are fabricated by means of welding, so they must be excellent in weldability, too. With respect to workability, it is important that the plates be satisfactory in elongation and bending. With respect to weldability, mechanical strength of the weld is, of course, the most important factor. But intergranular corrosion cracking caused by sensitization of the welds is especially significant. As has been observed in the foregoing, the materials for railroad vehicles must be provided with various characteristics simultaneously and must be more satisfactory than in the case of steels for general use. More specifically, materials for railroad vehicles are required to have excellent workability, considerably good work-hardening property (not more than 0.8 in yield ratio (yield-to-tensile strength ratio)) and excellent intergranular corrosion cracking resistance in the welds, in addition to being of high strength.
Although materials now in use for railroad vehicles are satisfactory in strength and workability, they incur problems in intergranular corrosion cracking in many cases. It is known that this cracking is localized to sensitized areas of the welding heat-affected zone, and runs along the grain boundaries. This means that this cracking is due to the high intergranular corrosion sensitivity of the material, whether it is caused by the pure intergranular corrosion or by the stress corrosion under the remaining welding strain. The reason why the conventional high strength strainless steels have high intergranular corrosion sensitivity is thought to be that the conventional high strength stainless steels contain 0.05-0.12% carbon in order to attain high strength and good workability, and are used in the cold-rolled state. That is, the fact per se that they contain high carbon and the fact that the high carbon content promotes intergranular corrosion susceptibility when the material is cold-rolled account for the high sensitivity to intergranular corrosion cracking of these steels.
The purpose of this invention is to provide a high strength stainless steel which is free from high sensitivity to intergranular corrosion cracking of welds, which is inherent in the conventional high strength stainless steels, and that is possessed of strength, workability, etc. higher than those of the conventional high strength stainless steels.
For this purpose, primarily with the aim to reduce the intergranular corrosion sensitivity of welds and sensitization of cold-rolled plates, we have carried out an extensive experimental study in order to ascertain whether the above-mentioned sensitivity and the sensitization can be reduced with high strength and good workability retained by reducing the carbon content and in its stead adding nitrogen, which strengthens the solid solution phase like carbon. We have found that by reducing the carbon content and adding nitrogen, sensitization of cold worked plates, that is, sensitivity to intergranular corrosion cracking of the conventional high strength stainless steel can be reduced while retaining strength and workability required for railroad vehicle materials, if the composition of the steel is adjusted with respect to stability of the austenite phase.